Master cylinder comprising a valve with reduced pedal free travel

ABSTRACT

This invention relates to a master cylinder ( 10 ), inside a bore ( 14 ) of which two axial primary and secondary pistons ( 16, 18 ), are slidingly fitted, and in which at least one sealing means ( 48, 50 ) is disposed in contact with each piston, defining a supply chamber ( 56, 60 ) and a pressure chamber ( 58, 62 ). Each piston ( 16, 18 ) comprises a bore ( 78, 80 ), open in the direction of the front pressure chamber ( 58, 62 ), and the piston ( 18 ) comprises a valve-forming through-hole ( 88 ), opening into its periphery and into its bore ( 80 ), in order to isolate the front pressure chamber when it passes beyond the sealing means so as to produce a braking pressure, characterised in that the front pressure chamber ( 62 ) associated with at least one piston ( 18 ) comprises at least one tubular cylindrical element ( 90 ) extending axially outside the piston ( 18 ) and rearwards of the sealing means ( 50 ) for the obturation of the through-hole ( 88 ) according to a reduced stroke (C m2 ).

[0001] This invention relates to a hydraulic-braking master cylinder ofthe “tandem” type for a motor vehicle.

[0002] More particularly, the present invention relates to ahydraulic-braking master cylinder of the “tandem” type for a motorvehicle, of the type comprising a substantially axial body, inside abore of which two axial pistons, namely a primary one and a secondaryone, are slidingly fitted in the back-to front direction respectively,and are capable of being actuated by the driver of the vehicle, betweena rest rear position and a forward position, in which a braking force isapplied; of the type in which each piston is resiliently returned to itsrest rear position onto a first rear stop; of the type in which at leastone sealing means is disposed between each piston and the bore so as todefine, within the bore, a rear hydraulic-fluid supply chamber and afront pressure chamber; of the type in which each piston comprises abore, open in the forward direction and communicating with the frontpressure chamber; of the type in which the piston comprises at least onesubstantially radial valve, opening into its periphery and into itsbore, and capable of moving between a let-through position, whichcorresponds to the rest rear position of the piston, and in which thevalve is situated rearwards of the sealing means and connects the rearsupply chamber with the front pressure chamber, and an isolatingposition, which corresponds to the forward application position of thepiston and in which the valve is situated forwards of the sealing meansand separates the rear supply chamber from the front pressure chamber,so as to produce a hydraulic braking pressure inside the front pressurechamber.

[0003] Numerous implementations of a master cylinder of said type arewell known.

[0004] In such a master cylinder, it is only once the associated pistonhas covered a determined distance, the so-called “dead stroke”, which isthe distance between the rest position of the radial valve of the pistonand the sealing means, that the hydraulic braking pressure may beproduced inside each pressure chamber.

[0005] As a matter of fact, as long as the radial valve is situatedrearwards of the sealing means, the rear supply chamber and the frontpressure chamber are communicating and under an equal pressure.

[0006] But when, as a result of the forward travel of the piston, theradial valve has moved past the sealing means, i.e. when the piston hascovered all the “dead stroke”, the front pressure chamber is separatedfrom the rear supply chamber and consequently the hydraulic brakingpressure increases inside the front pressure chamber as the piston ismoving forwards.

[0007] Therefore so as to produce, as quickly as possible, a hydraulicbraking pressure in the front pressure chamber taken into consideration,the “dead stroke” of the piston should be reduced as much as possible.

[0008] Now, a modification of the design dimensions of the mastercylinder could not possibly reduce the distance between the radial valveof the piston at rest and the sealing means.

[0009] As a matter of fact, on the one hand, at rest, the radial valvemust face a supply port, which communicates with a hydraulic-fluid tankfor the master cylinder.

[0010] On the other hand, the sealing means, generally in the form of alip seal accommodated inside a groove provided in the master cylinder,is situated away from the supply port, by at least a distancecorresponding to the thickness of a wall of the groove intended for theretention of the lip seal.

[0011] The document U.S. Pat. No. 4,989,498 discloses a front pressurechamber, associated with at least one piston, and comprising at leastone tubular cylindrical element outside the piston and retained axiallyin the body of the master cylinder, said cylindrical element extendingaxially rearwards of the sealing means and being capable, when thepiston travels forwards, of obturating the radial valve so as to obtaina closing travel of the valve which is smaller than the distance betweenthe valve and the sealing means.

[0012] Yet it is essential that the pressure chamber should be quicklyresupplied towards the end of a braking operation. But the mastercylinder, as described in U.S. Pat. No. 4,989,498, does not ensure sucha quick resupply.

[0013] Therefore, it is an object of the present invention to provide animproved means for the reduction of the “dead stroke”, in that anelement is interposed in order to seal off the radial valve before thelatter passes the sealing means.

[0014] To this end, the invention provides a master cylinder of theabove-described type, characterised in that a substantially annularspace for the resupply of the front pressure chamber is provided betweenthe body of the master cylinder and the sealing tubular element, saidspace connecting the front pressure chamber to the rear supply chamber.

[0015] According to other features of this invention

[0016] the sealing means consists of a lip seal, accommodated in agroove made in the bore of the master cylinder

[0017] the sealing means comprises an upper lobe, sealing off the spaceintended for the resupply of the front pressure chamber when thepressure, prevailing inside the front pressure chamber, is higher thanthat inside the rear supply chamber, and letting the brake fluid throughfrom the rear supply chamber into the front pressure chamber, when thepressure, prevailing inside the front pressure chamber, is lower thanthat inside the rear supply chamber;

[0018] the piston is made in a single piece, and the radial valvecomprises at least one through-hole opening into the periphery and intothe bore of the piston ;

[0019] the tubular cylindrical element consists of an annular ringhaving an inner diameter equal to that of the piston, said ring beingadjacent to the back face of the lip seal and being accommodated insidea complementary groove provided in the master cylinder;

[0020] the seal comprises a further lip, bearing on a front transverseface of the annular ring;

[0021] the ring is slit so as to facilitate its fitting at least fromthe rear part of the master cylinder;

[0022] the master cylinder comprises a front-end transverse face capableof being dismounted for the insertion of at least a solid ring and ofthe lip seal, from the front part of the master cylinder;

[0023] the master cylinder comprises a further lip seal, arrangedrearwards of a hydraulic-fluid feed channel which opens into the supplychamber of the piston, the ring being axially disposed between the lipseals;

[0024] the ring has a radial through-hole, facing the hydraulic-fluidfeed channel

[0025] each of the front- and rear-end transverse faces of the ringcomprises an annular groove for the accommodation of a complementaryflange for the retention of the adjoining lip seal.

[0026] Other features and advantages of the present invention will beapparent from the following detailed description, when taken inconjunction with the accompanying drawings, in which:

[0027]FIG. 1 is an axial sectional view of a master cylinder of the“tandem” type, according to a prior art and shown in the rest position;

[0028]FIG. 2A-2C are detail sectional views of a master cylinder of the“tandem” type according to a first embodiment of the present invention,and represented in the rest position, in the isolating position of thesecondary front pressure chamber and in the position in which thesecondary piston applies a braking force, respectively;

[0029]FIG. 3 and 4 are perspective views of two embodiments of the ringforming the tubular cylindrical element according to the firstembodiment of the invention;

[0030]FIG. 5A-5C are detail sectional views of a master cylinder of the“tandem” type according to a second embodiment of the present invention,and represented in the rest position, in the isolating position of thesecondary front pressure chamber and in the position in which thesecondary piston applies a braking force, respectively; and

[0031]FIG. 6 is a detail sectional view of a master cylinder of the“tandem” type according to a third embodiment of the present invention,and represented in the rest position of the secondary piston.

[0032] In the following description, the same reference numerals willdesignate the same elements, or elements having similar functions.

[0033] As a rule, the terms “front” and “rear” refer respectively toelements or positions facing leftward, or rightward in FIG. 1 through 6.

[0034]FIG. 1 is a general view of a braking master cylinder 10 for amotor vehicle.

[0035] In a well-known manner, the master cylinder 10 is a mastercylinder of the “tandem” type, which comprises a substantially axialbody 12 having an axis A and inside a bore 14 of which two axial pistons16 and 18 are fitted for a sliding travel.

[0036] The piston 16, the so-called primary piston 16, is intended toproduce a hydraulic pressure in a primary braking circuit of the vehicle(not shown), and the piston 18, which is the so-called secondary piston18, is designed to produce a hydraulic pressure in a secondary brakingcircuit of the vehicle (not shown), which is independent of the primarybraking circuit.

[0037] The primary piston 16 is capable of being directly actuated bythe driver of the vehicle. For instance, a rear end 20 of the primarypiston 16 may be connected to a servomotor (not shown) which amplifiesthe force exerted on a brake pedal of the vehicle.

[0038] The secondary piston 18 is capable of being indirectly actuatedby the driver of the vehicle, more particularly through the primarypiston 16, in a way to be explained hereafter.

[0039] Therefore, each piston, that is the primary piston 16 and thesecondary piston 18, is movable between a rest rear position,illustrated in FIG. 1, and a front position (not shown) in which abraking force is applied.

[0040] As is well known too, both the primary piston 16 and thesecondary piston 18 are resiliently returned to their rest rearpositions. Each piston may be resiliently returned to its rest rearposition so as to bear on a stop (not shown).

[0041] The secondary piston 18 is biased rearwards by means of a spring38, bearing on a front-end transverse face 40 of the bore 14 and on thesecondary piston 18, and the primary piston 16 is biased rearwards bymeans of a spring 42, bearing on a rear transverse face 44 of thesecondary piston 18 and on the primary piston 16. More particularly, thespring 42 is fitted about a sliding actuator 46, interposed between therear transverse face 44 of the secondary piston 18 and the primarypiston 16. The spring 42 is prestressed through the spring 38.

[0042] The master cylinder comprises at least two front sealing means,namely a primary one 48 and a secondary one 50, which are interposedbetween the primary piston 16 and the bore 14, and between the secondarypiston 18 and the bore 14, respectively.

[0043] In the preferred embodiment of the present invention, but by nomeans as a limitation, the primary and secondary sealing means consistof lip seals 48 and 50, accommodated in grooves 52 and 54 provided inthe body 12 and situated rearwards of shoulder-forming portions 28 and36 of the bore 14, respectively. Each seal 48, 50 comprises at least onelip, which is in contact with the periphery of the associated piston 16,18.

[0044] Of course, such arrangement should by no means be considered as alimitation imposed on the invention, and the seals 48, 50 might just aswell be received in cylindrical housings inserted in the body of themaster cylinder and comprising wholly or partly the grooves 52, 54accommodating the seals 48, 50.

[0045] The primary sealing means 48 defines, within the bore 14, aprimary rear supply chamber 56 and a primary front pressure chamber 58.In the same way, the secondary sealing means 50 defines, within the bore14, a secondary rear supply chamber 60 and a secondary front pressurechamber 62.

[0046] The body 12 comprises a primary radial feed channel 64 connectingan external primary hydraulic-fluid tank (not shown) to the primary rearsupply chamber 56.

[0047] More particularly, the primary radial channel 64 is connected,e.g. through an intermediate longitudinal channel 66, to a port 68opening outside the body 12 and intended to receive an outlet duct (notshown) of the associated primary tank.

[0048] In the same way, the body 12 comprises a secondary radial feedchannel 70 connecting an external secondary hydraulic-fluid tank (notshown) to the secondary rear supply chamber 60. This channel 70 opensoutside the body 12 through a port 72, which is intended to receive anoutlet duct (not shown) of the associated secondary tank.

[0049] The body 12 is provided with two through-holes, namely a primaryone 74 and a secondary one 76 which open, on the one hand, into theprimary and secondary pressure chambers, 58 and 62 respectively and, onthe other hand, to the outside of the body 12 so as to feed therespective primary and secondary braking circuits (not shown).

[0050] In a well-known manner, each piston 16, 18 comprises a bore 78,80, open in the forward direction and communicating with the frontpressure chamber 58, 62. Each piston 16, 18 comprises at least one valve82, 84 disposed between the rear supply chamber 56, 60 and the bore 78,80 of the piston 16, 18.

[0051] Each valve 82, 84 comprises, by way of example and by no means asa limitation, at least one radial through-hole 86, 88 provided betweenthe bore 78, 80 of each piston 16, 18 and its periphery, and theposition of which in relation to the lip seal 48, 50 either permits orprohibits the flow of hydraulic fluid from the rear supply chamber 56,60 towards the front pressure chamber 58, 62.

[0052] Thus, each valve 82, 84, is capable of moving between an openposition, illustrated in FIG. 1, in which, when one of the pistons 16,18 is in its rest rear position, it connects the rear supply chamber 56or 60 of said piston 16, 18 with the front pressure chamber 58 or 62 ofsaid piston 16, 18, and a closed position (not shown) in which, when thepiston 16, 18 is moved axially forwards towards its applicationposition, it separates the respective front pressure chamber 5.8, 62from the respective rear supply chamber 56, 60 so as to produce abraking pressure inside the front pressure chamber 58, 62.

[0053] In the subsequent description, it is the mode of operation of thesecondary piston 18 in the secondary front pressure chamber 62 whichwill be dealt with in greater detail, considering that the primary andsecondary pistons 16, 18 operate in a substantially similar manner.

[0054] More particularly, in the above-mentioned arrangement, theclosing of the secondary valve 84 can only occurs when the secondarypiston 18 has moved by a stroke “C_(m1)”, that is the so-called “deadstroke”, which corresponds to the distance between the through-hole 88and the seal 50.

[0055] Thus, as long as the piston 18 has not covered the dead stroke“C_(m1)”, the through-hole 88 remains rearwards of the lip seal 50 andthe hydraulic fluid can flow from the secondary rear supply chamber 60into the secondary front pressure chamber 62. In this case, the valve 84is open.

[0056] On the other hand, once the piston 18 has covered the dead stroke“C_(m1)”, the through-hole 88 has passed the lip seal 50 and thehydraulic fluid flow from the secondary rear supply chamber 60 into thesecondary front pressure chamber 62 is stopped. In this case, the valve84 is closed.

[0057] Now, in order to benefit from a highly efficient brakingoperation, it is essential that the value of the dead stroke should bereduced as much as possible.

[0058] But, in the above-described arrangement, the distance between thethrough-hole 88 and the seal 50 cannot be reduced at all.

[0059] As a matter of fact, on the one hand, a rear wall 53 of thegroove 54 must be thick enough for the adequate retention of the seal 50inside the associated groove 54. And, on the other hand, in the restposition of the piston 18, the through-hole 88 must substantially facethe feed channel 70 so as to obtain an optimum supply of the valve 84with hydraulic fluid.

[0060] So as to eliminate such a disadvantage, the present inventionprovides a master cylinder of the above-described type, characterised inthat the front pressure chamber 62, associated with at least one piston16, 18, comprises at least one tubular cylindrical element, which isdisposed outside the piston 16, 18 and extends axially rearwards of thesealing means and which is capable, when the piston 16, 18 travelsforwards, of obturating the radial valve for the valve-closing deadstroke to be smaller than the distance between the valve and the sealingmeans.

[0061] Therefore and still referring to the operating conditions of thesecondary piston, as illustrated in the figures, the master cylinder ischaracterised in that the front pressure chamber 62, associated with thesecondary piston 18, comprises at least one tubular cylindrical element90 which is disposed outside the piston 18 and extends axially rearwardsof the sealing means 50 and which is capable, when the secondary piston18 travels forwards, of obturating the radial valve 84 so as to obtain adead stroke “Cm₂l” for the closing of the valve 84, which is smallerthan the distance between the valve 84 and the sealing means 50.

[0062]FIG. 2A through 2C and 5A through 6 illustrate several embodimentsof a master cylinder 10 comprising such a tubular cylindrical element90.

[0063] According to a first embodiment of this invention, as shown inFIG. 2A through 2C, the secondary piston 18 is made in a single piece,and the radial valve 84 comprises at least one through-hole 88 openinginto the periphery and into the bore 80 of the secondary piston 18.

[0064] Here, the tubular cylindrical element 90 consists of an annularring having an inner diameter equal to the diameter “D₁₈” of the piston18, said ring being adjacent to the back face of the lip seal 50 andbeing accommodated inside a complementary groove 120 provided in thebody 12 of the master cylinder 10.

[0065] The outer diameter of the annular ring is smaller than the innerdiameter of the groove 120, thus providing a substantially annular space51 connecting the pressure chamber 62 to the supply chamber 60.

[0066] Of course, such an arrangement is by no means limitative of theinvention and the annular ring 90 might just as well be received in acylindrical housing inserted in the body of the master cylinder saidhousing comprising wholly or partly the groove 120 accommodating thering 90.

[0067] In this embodiment, the seal 50 comprises a lip 122 in awell-known manner, and besides it also comprises a further rear lip 124,bearing on the periphery of the piston 18 and moreover on a fronttransverse face 126 of the annular ring 90. The latter lip provides forthe tightness relative to the hydraulic fluid between the seal 50 andthe annular ring 90, when the through-hole 88 is situated axially atright angles with the face 126 of the annular ring 90. Furthermore, theseal comprises a front lobe 125, securing the seal 50 inside the groove54, and an upper lobe 127, which exhibits the adequate flexibility forthe passage of the hydraulic fluid through the annular space 51, whichconstitutes the resupply channel between the body of the master cylinderand the radially outer periphery of the ring, so as to resupply thefront chamber 62 when the pressure, prevailing inside this frontpressure chamber 62, is lower than that inside the supply chamber 60.The annular space is sealed off by the upper lobe 127 when the pressure,prevailing inside the pressure chamber 62, is higher than that insidethe supply chamber 60.

[0068] The ring 90 is locked lengthwise along the axis A, by means ofaxial stops, formed in the rearward direction by an annularshoulder-forming portion at the junction of the groove 120 and thechannel 70 and, in the forward direction, in this example, by the seal50 which is in turn kept in position by the lobe 125.

[0069] In order to keep the ring 90 in position in the forwarddirection, a receiving groove for the ring 90 may also be provided inthe groove 120, the former having a greater diameter that the groove120. Therefore the ring 90 would bear by its front part against ashoulder. In this case, passages should also be provided for the fluidflow from the annular space towards the pressure chamber 62, forinstance in the outer periphery of the ring 90 or in the shoulder,forming the front stop for the ring 90.

[0070] The ring may be made according to various designs. Preferably, asshown in FIG. 3, the ring 90 may be slit so as to facilitate its fittingfrom the rear part of the master cylinder 10 and to ensure a leakprooftightening on the piston 18.

[0071] The annular ring 90 may be inserted using an adequate sleeve (notshown), and therefore in a somewhat deformed state, in the bore 14 afterthe positioning of the seal 50 and prior to the introduction of thepistons 16, 18 into the bore 14. Then the sleeve is removed and theannular ring relaxes spontaneously as it enters the groove 120.

[0072] As a variant, the master cylinder 10 may comprise a front-endtransverse face (not shown) capable of being dismounted for theinsertion of at least a solid ring 90, like the one illustrated in FIG.4, and of the lip seal 50, from the front part of the master cylinder10.

[0073] Said dismountable front-end transverse face can also permit theintroduction of a slit ring 90, and/or of the seals 48 and 50, and ofthe rings, associated with both pistons 16 and 18.

[0074] Consequently, the piston 18 is capable of moving between threecharacteristic positions.

[0075] The dead stroke “C_(m2)” is reduced to the distance between thethrough-hole 88 and a back face 128 of the ring, constituting theelement 90, as shown in FIG. 2A.

[0076] Initially, the secondary piston 18 is in its rest position, asillustrated in FIG. 2A. In this configuration, the valve 84 is openbecause the through-holes 88 are situated rearwards of the back face 128of the tubular element-forming ring 90 and therefore they are not cappedwith such ring 90.

[0077] Afterwards and as illustrated in FIG. 2B, the secondary piston 18moves forwards under the action of a force transmitted by the returnspring 42 of the primary piston 16 and therefore the valve 84 closesbecause the through-hole 88 is covered now with the tubularelement-forming ring 90, with the result that the hydraulic fluid flowis interrupted between the secondary rear supply chamber 60 and thesecondary front pressure chamber 62, which means that the dead stroke“C_(m2)” has been travelled by the secondary piston 18 and consequentlya hydraulic braking pressure can be produced inside the secondary frontpressure chamber 62.

[0078] And ultimately, in a more advanced position of the secondarypiston 18, as represented in FIG. 2C, the through-hole 88 has passed theseal 50, thus producing an increasing braking pressure, and then thepiston 18 in an end-of-stroke position abuts against the front-endshoulder-forming face 40 of the bore 14 of the master cylinder 10.

[0079] According to a second embodiment of the present invention, to bedescribed hereunder with reference to FIG. 5A through 5C, the mastercylinder comprises, in particular, a lip seal 130 arranged rearwards ofthe hydraulic-fluid feed channel 70 which opens into the supply chamber60 of the secondary piston 18, and the ring 90 is axially disposedbetween the lip seals 50 and 130.

[0080] More particularly, the ring 90 has at least one radialthrough-hole 132, facing a groove connected to the hydraulic-fluid feedchannel 70.

[0081] In this configuration, a further function of the ring 90 consistsin maintaining the seals 50 and 130.

[0082] The ring 90 comprises a front annular section 134, the innerdiameter of which is equal to that of the piston 18, said section beingcontiguous to the back face of the lip seal 50, whereas the outerdiameter of the annular section 134 is smaller that the diameter of thegroove accommodating the section 134, so as to provide an annular space51 which constitutes a resupply channel for the front pressure chamber62 from the rear supply chamber 60. The lip seal 50 is identical withthat of the preceding embodiment and therefore it comprises two lips 122and 124. The ring 90 also comprises a rear annular section 135 having aninner diameter which is equal to that of the piston 18.

[0083] The ring 90 is held in position lengthwise in the body of themaster cylinder through a projection, surrounding the through-hole 132in conjunction with the hydraulic-fluid feed channel 70.

[0084] Such projection is e.g. either an annular projection or a stud,the outer diameter of which is substantially equal to the diameter ofthe channel 70. Passages (not shown) for the flow of the hydraulic fluidfrom the channel 70 into the annular space are made, for instance, inthe projection of the ring 90 or in the body of the master cylinder.

[0085] Consequently, the piston 18 is capable of moving between threecharacteristic positions.

[0086] The dead stroke “C_(m2)” is reduced to the distance between thethrough-hole 88 and a rear transverse face 136 of the front section 134of the tubular element-forming ring 90, as shown in FIG. 5A.

[0087] Initially, the secondary piston 18 is in its rest position, asillustrated in FIG. 5A. In this configuration, the valve 84 is openbecause the through-holes 88 are situated rearwards of the rear face 136of the front section 134 of the tubular element-forming ring 90.

[0088] Afterwards and as illustrated in FIG. 5B, the secondary piston 18moves forwards under the action of a force transmitted by the returnspring 42 of the primary piston 16 and therefore the valve 84 closesbecause the through-hole 88 is covered now with the front section 134 ofthe tubular element-forming ring 90, with the result that the hydraulicfluid flow is interrupted between the secondary rear supply chamber 60and the secondary front pressure chamber 62, which means that the deadstroke “C_(m2)” has been travelled by the secondary piston 18 andconsequently a hydraulic braking pressure can be produced inside thesecondary front pressure chamber 62.

[0089] And ultimately, in a more advanced position of the secondarypiston 18, as represented in FIG. 5C, the through-hole 88 has passed theseal 50, thus producing an increasing braking pressure, and then thepiston 18 in an end-of-stroke position abuts against the front-endshoulder-forming face 40 of the bore 14 of the master cylinder 10.

[0090] At the end of the braking operation, if the pressure prevailinginside the pressure chamber 62 is lower that the pressure inside thesupply chamber 60, the resupply of the pressure chamber 62 is carriedout in the same way as for the first embodiment, represented in FIG. 2Athrough 2C.

[0091] It should be noted that, according to a variant illustrated inFIG. 6, each one of front- and rear-end transverse faces 138 and 140 ofthe ring 90 may comprise an annular groove 142, 144 accommodating acomplementary flange 146, 148 for the retention of the adjoining lipseal 50, 130. Said flange 146, 148 may be either press fitted or stuck.

[0092] The sealing-off of the through-holes 88 by the ring 90 means thebeginning of the pressure rise inside the pressure chamber 62 but,however, it is only when the through-holes 88 have passed the seal 50that the high pressure, which is required for the braking action, isavailable.

[0093] Such an arrangement is by no means limitative of the invention,and the front- and rear-end transverse faces 138 and 140 of the ring 90may exhibit any suitable shape to hold down a complementary fasteningmeans for the retention of the adjoining lip seal 50, 130, suchfastening means being not necessarily a flange.

[0094] In an advantageous manner, this configuration makes it possibleto fit the ring 90 and both seals 50 and 130 all in one block and,accordingly, to dispense with a lip 124 of the seal 50, such as theabove-described one.

[0095] Therefore the invention provides a master cylinder 10 having areduced stroke, which is the token of an improved braking safety.

We claim:
 1. A hydraulic braking master cylinder (10) of the “tandem”type for a motor vehicle, of the type comprising a, substantially axialbody (12), inside a bore (14) of which two axial pistons, namely aprimary piston (16) and a secondary piston (18), are slidingly fitted inthe back-to front direction respectively, and are capable of beingactuated by the driver of the vehicle between a rest rear position and aforward position, in which a braking force is applied; of the type inwhich each piston (16, 18) is resiliently returned to its rest rearposition onto a first rear stop; of the type in which at least onesealing means (48, 50) is disposed between each piston (16, 18) and thebore (14) so as to define, within the bore (14), a rear hydraulic-fluidsupply chamber (56, 60) and a front pressure chamber (58, 62); of thetype in which each piston (16, 18) comprises a bore (78, 80), open inthe forward direction and communicating with the front pressure chamber(58, 62); of the type in which the piston (16, 18) comprises at leastone substantially radial valve (82, 84), opening into its periphery andinto its bore (78, 80), and capable of moving between a let-throughposition, which corresponds to the rest rear position of the piston (16,18) and in which the valve (82, 84) is situated rearwards of the sealingmeans (48, 50) and connects the rear supply chamber (56, 60) with thefront pressure chamber (58, 62), and an isolating position, whichcorresponds to the forward application position of the piston (16, 18)and in which the valve (82, 84) is situated forwards of the sealingmeans (48, 50) and separates the rear supply chamber (56, 60) from thefront pressure chamber (58, 62), so as to produce a hydraulic brakingpressure inside the front pressure chamber (58, 62), wherein thepressure chamber (62), associated with at least one piston (18)comprises at least one tubular cylindrical element (90) outside thepiston and retained axially in the body (12) of the master cylinder,said cylindrical element (90) extending axially rearwards of the sealingmeans (50) and being capable, when the piston (18) travels forwards, ofobturating the radial valve (84) so as to obtain a closing travel(C_(m2)) of the valve which is smaller than the distance between thevalve (84) and the sealing means (50), characterised in that asubstantially annular space (51) for the resupply of the front pressurechamber (58, 62) is provided between the body of the master cylinder andthe sealing tubular element (90), said space connecting the frontpressure chamber (58, 62) to the rear supply chamber (56, 60).
 2. Themaster cylinder (10) according to the preceding claim, characterised inthat the sealing means (50) consists of a lip seal, accommodated in agroove (54) made in the bore (14) of the master cylinder (10).
 3. Themaster cylinder (10) according to the preceding claim, characterised inthat the sealing means (50) comprises an upper lobe, sealing off thespace (51) intended for the resupply of the front pressure chamber (58,62) when the pressure, prevailing inside the front pressure chamber (58,62), is higher than that inside the rear supply chamber (56, 60), andletting the brake fluid through from the rear supply chamber (56, 60)into the front pressure chamber (58, 62), when the pressure, prevailinginside the front pressure chamber (58, 62), is lower than that insidethe rear supply chamber (56, 60).
 4. The master cylinder (10) accordingto claim 2, characterised in that the piston (18) is made in a singlepiece, and in that the radial valve comprises at least one through-hole(88) opening into the periphery and into the bore (80) of the piston(18).
 5. The master cylinder (10) according to the preceding claim,characterised in that the tubular cylindrical element (90) consists ofan annular ring having an inner diameter equal to that of the piston(18), said ring being adjacent to the back face of the lip seal (50) andbeing accommodated inside a complementary groove (120) provided in themaster cylinder (10).
 6. The master cylinder (10) according to thepreceding claim, characterised in that the seal (50) comprises a furtherlip (124), bearing on a front transverse face (126) of the annular ring(90).
 7. The master cylinder (10) according to the preceding claim,characterised in that the ring (90) is slit so as to facilitate itsfitting at least from the rear part of the master cylinder (10).
 8. Themaster cylinder (10) according to claim 6, characterised in that itcomprises a front-end transverse face capable of being dismounted forthe insertion of at least a solid ring (90) and of the lip seal (50),from the front part of the master cylinder.
 9. The master cylinder (10)according to claim 8, characterised in that it comprises a further lipseal (130), arranged rearwards of a hydraulic-fluid feed channel (70)which opens into the supply chamber (60) of the piston (18), and in thatthe ring (90) is axially disposed between the lip seals (50, 130). 10.The master cylinder (10) according to the preceding claim, characterisedin that the ring (90) has a radial through-hole (132), facing thehydraulic-fluid feed channel (70).
 11. The master cylinder (10)according to claim 10, characterised in that each of the front- andrear-end transverse faces (138, 140) of the ring (90) comprises anannular groove (142, 144) accommodating a complementary flange (146,148) for the retention of the adjoining lip seal (90, 130).